Technical Field
The present disclosure relates to a power module that achieves electrical energy conversion by electrically connecting semiconductor power chips.
Description of Related Art
In industrial products, power modules are widely used as core components for electrical energy conversion. Semiconductor power chips packaged in the power modules mainly include IGBTs (Insulated-Gate Bipolar Transistor), MOSFETs (Metal-Oxide Semiconductor Field Effect Transistor), FRDs (Fast Recovery Diode), and etc. High reliability of the power modules is a major goal that has been pursued continuously during designing of the modules, which means long-life time and low maintenance cost for those industrial system-level products.
FIG. 13A shows a cross-sectional view of a conventional power module. A power chip 91 of the power module is bonded to a DBC (Direct Bonded Copper) substrate 90 by solder 92. CTEs (Coefficient of thermal expansion) of the power chip 91 and the DBC substrate 90 are 4.1×10−6 m/° C. and 7×10−6˜9×10−6 m/° C. respectively, which are highly mismatched. When the power module works, the power chip 91 will conduct switching operations. As there is an internal resistance in the power chip 91, a large amount of on-state loss will be generated when a large current flows through the power chip 91 under on-state condition. Meanwhile, a large amount of switching loss will be generated during the switching operations by the power chip 91. The total loss comprised by the on-state loss and switching loss above will result in a large amount of heat, and thus the temperature of the power chip 91 will increase. High thermal stresses will be generated at the solder 92 due to large CTE mismatches between the power chip 91 and the DBC substrate 90 when temperature increasing. In addition, the power module may be operated in different modes, such as a light-load mode, a full-load mode, an overload mode, or even off-state mode. The temperature of the power chip 91 will change frequently when switched between those modes. Therefore, the solder 92 continuously will withstand the cyclic thermal stresses. Under long working hours, creep and thermal mechanical fatigue will occur in the solder 92 under long-time operation, resulting in the cracks 920 and final failure of the solder 92.
In the conventional power module, the power chip 91 is bonded onto the DBC substrate 90 by the solder 92 (e.g. solder paste with organics before soldering process) with the vacuum reflow soldering technology, achieving a sandwich structure. During soldering, the solder 92 melts, and organic solvents in the solder 92 are continuously vaporized to lift the power chip 91 up. Therefore, it is easy to make the thickness of the solder 92 be nonuniform and thus the power chip 91 tilts, as shown in FIG. 13B. A larger thermal stress occurs at the location with a smaller thickness of the solder 92. The thinner parts of the solder 92 are easily to be cracked, further resulting in fatigue failure of the solder 92. In order to ensure the lifetime of the solder 92 underneath the power chip 91, a solution to prolong the lifetime of the solder 92 is increasing the overall thickness of the solder 92, which will ensure the part with the smallest thickness of the solder 92 meeting the reliability requirement. However, more solder 92 will be employed, which will waste materials and increase the material costs. In addition, the thermal resistance of the power module will be increased by thicker average thickness of the solder 92, resulting in the deterioration of the thermal performance of the power module. Another solution is using novel bonding materials and technologies, such as LTJT (Low-Temperature Joining Technology), diffusion soldering, etc. Due to good thermo-mechanical performances of the novel technologies, the lifetime of the sandwich structure can be improved. However, the technologies need to adopt novel materials and equipments, which will increase the cost of materials and packaging processes.
Accordingly, a cost-effective method is highly required to improve the reliability of the bonding material 92 in a power module.